Since plastic materials and woodworking materials including engineering plastics are light in weight and have moldability and tenacity, they have recently been used as base materials for machines, general industrial products and daily miscellaneous goods, mainly for electric appliances. However, base materials such as plastic materials and wood materials have low hardness and poor abrasion resistance and are therefore frequently provided with a surface protective layer by applying and curing a coating material on a surface of the base material.
In order to provide the surface protective layer, for example, a coating agent containing a radiation-curable resin composition which is cured by irradiation of ultraviolet rays or electron beams (radiation-curable coating material) has been put into practical use in view of such advantages as energy saving, improvement in workability and improvement in productivity upon formation of the surface protective layer. However, in the radiation-curable coating material which forms a protective layer having high hardness, deformation of a base material and cracking of a cured coating film are likely to occur due to shrinkage upon curing (curing shrinkage), and thus a radiation-curable coating material, which gives a cured coating film having high hardness and low curing shrinkage, is required.
As the radiation-curable coating material which gives a cured coating film having high hardness and low curing shrinkage, for example, a resin composition for radiation-curable coating material is disclosed, which is obtained by reacting a (meth)acrylate compound having a hydroxyl group and two or more (meth)acryloyl groups with an aliphatic polyisocyanate compound in a molar ratio (OH/NCO) of a hydroxyl group to an isocyanate group within a range from 1.01 to 1.24, and also contains a urethane (meth)acrylate compound having 5 or more (meth)acryloyl groups on average (for example, refer to Japanese Unexamined Patent Publication, First Publication No. 2002-241646 (“JP '646”)). Although a cured coating film having high hardness is obtained from a radiation-curable coating material using the resin composition for radiation-curable coating material disclosed in Patent Document 1, there is a problem that, when the cured coating film is formed on a thin base material such as a film, the coating film exhibits large curing shrinkage and deformation such as curl of the base material occurs.